/Engine/Source/Runtime/GeometryCore/Public/DynamicMesh/DynamicMesh3.h
If edge has vertices [a,b], and is connected two triangles [a,b,c] and [a,b,d], this returns [c,d], or [c,InvalidID] for a boundary edge
[FIndex2i](API\Runtime\GeometryCore\FIndex2i) GetEdgeOpposingV ( int EdgeID ) const